Propylene resins are used in a various kinds of fields including miscellaneous daily goods, kitchen goods, packaging films, home electric appliances, machine parts, electric parts, and automotive parts. Propylene-based resin compositions containing various modifiers and additives according to required performance are used. Additionally, in order to promote three Rs (Reduce, Reuse, and Recycle) for forming a recycling-oriented society, attempts for weight reduction by thinning molded articles have recently been made in individual industrial fields. Propylene-based resin compositions are being further improved so that even light-weighted and thin molded articles can have sufficient rigidity and impact resistance.
In general, a modifier added to a propylene resin is a soft olefin-based resin made of an ethylene/α-olefin-based copolymer. For further performance improvement, it is expected to apply, as a modifier, an olefin-based block polymer prepared by chemically bonding a crystalline polyethylene segment to a noncrystalline or low-crystalline ethylene/α-olefin-based copolymer segment.
Examples of techniques relating to such an olefin-based block polymer include a technique relating to a linear block polymer including a polyethylene segment and an ethylene/α-olefin-based copolymer segment obtained by using a living polymerization catalyst disclosed in Patent Literature 1 and a technique relating to production of a multi-block polymer using a reversible chain-transfer reaction between two kinds of catalysts disclosed in Patent Literature 2.
Apart from such linear block polymers, Patent Literature documents 3 to 8 have proposed methods for obtaining grafted copolymers having a main chain and one or more side chains and composed of heterogeneous composition segments, one of which is a soft segment and the other one is a hard segment. Disclosures of these methods are generally based on a technique in which a hard segment such as a polyethylene having a terminal vinyl group is synthesized, and, following or simultaneously with the synthesis, the hard segment such as polyethylene is copolymerized with ethylene or an α-olefin having 3 or more carbon atoms to be introduced into the soft segment as the main chain.
For example, Patent Literature documents 3 and 4 disclose methods for obtaining a grafted olefin polymer by copolymerizing a vinyl-terminated polyethylene produced using a specific metallocene catalyst with ethylene. In the disclosed methods, while polyethylene having a terminal vinyl group is obtained, terminal vinyl production efficiency is low, as a result of which there remains a large amount of polyethylene not introduced as side chains. Adding such a grafted polymer into a polypropylene resin deteriorates mechanical physical properties such as impact resistance due to the large amount of contained nonreactive polyethylene. Accordingly, the performance of the grafted polymer as a modifier resin still needs to be improved. Thus, the use of the grafted polymer has not allowed for production of a polypropylene resin composition that exhibits intended physical properties.
Meanwhile, Patent Literature documents 5 to 7 have disclosed techniques for synthesizing a vinyl-terminated polyethylene for a side chain with high production efficiency by using a specific non-metallocene-based complex catalyst. The present inventors performed a follow-up investigation by using a catalyst for main chain production disclosed in Examples of the Patent Literature document 5 or 7 and confirmed that although a certain amount of vinyl-terminated polyethylene is copolymerized into the main chain, productivity is low in high-temperature conditions for introducing the vinyl-terminated vinyl polyethylene with high efficiency. In a block polymer thus obtained, an amount of side chains to be introduced is also limited. Accordingly, even when trying to use the block polymer as a modifier resin for polypropylene, a modification performance thereof is insufficient.
Patent Literature 8 has disclosed a technique for introducing a side chain with high efficiency by a co-supported catalyst system. However, because the main chain is limited to a crystalline polymer, it has been difficult to produce a polymer for a non- or low-crystalline region suitable to modify a propylene-based resin.
Thus, even with the use, as a modifier for a polypropylene resin, of the block polymers and the grafted polymers composed of chemically bonded heterogeneous polymers disclosed in the known techniques, there has been obtained no propylene-based resin composition that is highly balanced in rigidity and impact resistance.